Switch this BinaryBoxProtocol's transport to a new protocol. You need
to do this 'simultaneously' on both ends of a connection; the easiest way
to do this is to use a subclass of ProtocolSwitchCommand.

bytes received, but not yet broken up into messages / sent to
stringReceived. _compatibilityOffset must be updated when this value is
updated so that the recvd attribute can be generated
correctly. (type: bytes)

Temporary storage for all file descriptors received. Values in this
dictionary are the file descriptors (as integers). Keys in this dictionary
are ordinals giving the order in which each descriptor was received. The
ordering information is used to allow Descriptor to
determine which is the correct descriptor for any particular usage of that
argument type. (type: dict)